Visual contents, be they still or moving images, are in general creations which benefit from copyright-related exclusivity guarantees. Their reproduction is in general permitted only within a strictly defined framework which allows for remuneration of authors and their beneficiaries.
In order to ensure that these legal rules are correctly adhered to, numerous systems have been developed to prevent illegal copies or sufficiently impair their quality as to render them unusable.
Within this framework, patent application EP 1 237 369 aims to combat the copying of images by picture-taking during their display, for example with a camcorder in a cinema auditorium. With this aim, it is proposed that the intensity of the pixels of a pattern be modulated about the value to be displayed at a high frequency which renders the pattern invisible to the human eye but which generates artefacts on the sequence filmed by the camcorder. This pattern is commonly called a watermark or anti-copy pattern.
The shape of the pattern is determined so as to inscribe for example messages of the type “ILLEGAL COPY” which will appear in the images displayed by the camcorder.
In order for the pattern to be invisible to the naked eye, the modulation consists in alternating images in which the pattern is bright with images in which it is dark, the mean intensity of the pattern over several images corresponding to that to be displayed in the images in the absence of a pattern. During the display of these images, the eye carries out an integration and in fact perceives the mean intensity.
This technique poses a problem however when the images represent a scene in motion. Specifically, as the eye tends to follow the motion within the image, the temporal integration is no longer done correctly and the pattern then appears to the naked eye. Let us take the example of a modulation creating a deficit of luminance for a pixel P of the pattern in a first image and a complementary surplus of luminance for the same pixel in a second image. If the eye does not shift, it sums the luminances of these two pixels and then perceives the mean luminance value. The perception of the eye is then correct. If the eye shifts, the pixel P in the first image is not integrated by the same retinal zone of the eye as this same pixel in the second image. The visual sum between these two pixels is no longer correct and the pattern appears.